staging:iio: Setup buffer access functions when allocating the buffer

Setup the buffer access functions in the buffer allocate function. There is no
need to let each driver handle this on its own.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>